Water detection units are commonly employed to detect and measure a presence of undissolved water within a fuel supply. One common use of water detection units is in the aviation industry, for detecting and measuring undissolved water in jet fuel.
One common test for measuring undissolved water in jet fuel involves passing a measured sample of fuel through an absorbent filter pad, wherein an upstream side of the filter pad is coated with a uranine dye. See MIL-DTL-81248, for example. The filter pad is then illuminated using an ultraviolet (UV) light, wherein a presence of undissolved water on the coated side of the pad will cause the pad to fluoresce a bright yellow. The fluorescence of the pad is directly related to an amount of undissolved water present in the pad, wherein a greater amount of undissolved water results in a brighter fluorescence. Using a photocell or similar comparator, the UV-illuminated pad is then directly or indirectly compared to a reference standard having a known emittance, and the amount of undissolved water in the sample is determined.
In order to accurately determine an amount of undissolved water in a fuel sample, the water detection unit must be calibrated prior to use. To calibrate the water detection unit, a filter pad having a known amount of fluorescence, known as a reference standard or calibration standard, is loaded into the detection unit and subjected to UV light. A readout of the water detection unit is then adjusted until an indicated emittance matches a known emittance of the reference standard.
A common method for manufacturing reference standards involves providing a paper filament pad. One side of the paper filament pad is then treated with a fluorescent material. However, reference standards are difficult to manufacture, as substances that will absorb and emit UV light within the same spectrum of visible light are rare. Known organic substances, like unreacted uranine, are unstable and unlikely to provide repeatable and reproducible readouts over a period of time, while inorganic substances tend to not follow the same emittance curves as the test material, uranine.
As testing has evolved from being a simple visual test, to using sophisticated photocells and sensors, the use of inorganic materials having a different emittance curve than uranine has become problematic, as it is more likely to result in inaccurate calibration of the water detection unit. Particularly, older testing units were relatively insensitive to wavelengths of light, wherein dyes having different wavelengths could be used to calibrate the detection unit so long as the emittance was comparable to that of the uranine used on the actual test pad. However, modern detection units are highly sensitive to the wavelength of the emittance. Thus, the emittance of the reference standard must have a wavelength similar to the emittance of the test pads. Accordingly, conventional dyes are no longer reliable in a reference standard, as the wavelengths are different than the uranine used on the test pads.
Additionally, the fluorescents used in many reference standards have a high level of persistence, wherein the emittance of the material continues for an extended period of time after exposure of the fluorescent to UV light. High persistence may have an effect on subsequent readings when taken quickly in series, as residual emittance of the fluorescent may be erroneously included in the subsequent reading.
It is therefore desirable to provide a means of manufacturing a reference standard using a stable material that follows an emittance curve comparable to the emittance curve of uranine.